Zoonoses disease-salmonellosis, Biology

Salmonellosis

Salmonellosis is an infection caused by bacterial species belonging to the genus Salmonella and usually associated with the ingestion of food containing salmonellae or their products. Salmonellae are widely distributed in nature infecting man and animals alike and are major public health problems all over the world. Most of the Salmonella serotypes incriminated in gastroenteritis in man have been isolated from animals and birds.

Epidemiology: Some 2,500 serovars of Salmonella enterica are known. On the basis of host adaptability, these serovars are broadly divided into three groups:

1. Salmonellae adapted to man, e.g., S. Typhi, S. Paratyphi A, B and C and S.Sendai.2. Salmonellae adapted to animals, e.g., S. Choleraesuis, S. Gallinarum, S.Abortusequi and S. Abortusovis and S. Dublin.3. All other salmonellae are unadapted and produce infections equally in man in animals, e.g., S. Enteritidis, S. Typhimurium, etc.Salmonellae organisms are mostly transmitted by oral route through contaminated food or water. The incidence of salmonellosis is more in the summer months. All age groups are infected but the infection is more severe in elderly age group.

Clinical features: Three clinically distinguishable forms of salmonellosis occur in man enteric fever, septicaemia and acute gastroenteritis.Typhoid fever is the prototype of enteric fevers. The incubation period is 7-14 days. There is malaise, anorexia and headache followed by the onset of fever. Fever often increases in a step-like manner and is accompanied by brandycardia. Diarrhoea is usually absent, but abdominal tenderness and distension are common. Cough and signs of bronchitis may be present. Splenomegaly and leucopenia are common. In more severe cases the sensorium is dull and the patient becomes delirious. In fatal cases the most prominent lesion found at autopsy is lymphoid hyperplasia, ulceration of Peyer’s patches or perforation of bowel. Fever continues for 3 to 4 weeks.Enteric fevers (paratyphoid fevers) are usually milder and have a shorter incubation period (1-10 days). Fever usually lasts for 1-3 weeks.

Salmonella septicaemia are characterized by high, remittent fever and bacteraemia without apparent involvement of the gastrointestinal tract. Focal suppurative lesions may develop almost anywhere in the body including the biliary tract, kidneys, heart, spleen, meninges, joints and lungs.Gastroenteritis is a form of a disease confined to the gastrointestinal tract and is the most common form of Salmonella infection. Symptoms begins from 8 - 48 hours after the consumption of contaminated foods. Mild to severe diarrhoea ensues with sudden and violent onset. Headache, chills and abdominal pain are followed by nausea and vomiting accompanied by fever lasting from 1-4 days.

Laboratory diagnosis: A diagnosis of Salmonella infection is made by isolation of the organisms. Specimens of blood, faeces, vomit and any suspected food-stuff are used for isolation of the organism. Serological tests for specific antibody (Widal test) is performed on at least two serum specimens, the first obtained as early as possible and the second 7-10 days later. Nucleic acid based diagnosis like PCR may be used for rapid and accurate diagnosis.

Control and prevention: As a large number of unadapted Salmonella species are incriminated in salmonellosis , it is not practically possible to resort to immunoprophylaxis against even of the common serotypes. Since many Salmonella serotypes of animal origin are transmitted from animals to man, it is necessary to take suitable measures for the control of salmonellosis in animals. Moreover, necessary steps may be taken for destruction of Salmonella in food products and for prevention of their subsequent contamination by taken suitable hygienic precautions. Pasteurization of milk should be done. Provision of safe water and taking of hot usually prevent Salmonella infection. Detection of carrier food handlers and their treatment should also be carried out.